Organic electroluminescent devices (or diodes) are devices, where organic molecules emit light when a driving voltage is applied to such organic electroluminescent devices (OLEDs). The OLEDs typically comprise a transparent substrate with an electroluminescent layer stack deposited on top of the substrate comprising an organic light emitting layer stack between two electrode layers, typically a transparent anode on top of the substrate and a reflective cathode on top of the organic layer stack. Since the organic molecules are sensitive to moisture and oxygen, the layer stack is encapsulated by a gastight cover lid sealed on top of the substrate. In order to operate the OLED, driving voltages in the order of a few volts, e.g. 2-15 V are applied.
OLED devices are large area light sources intended for room illumination. However, the mount of light emitted from the OLED devices is only 20% of the amount of light internally generated by the light emitting layer. To increase the efficiency of the OLED devices, an improved light out-coupling is desired. The major loss factor in OLED devices is trapping of the light due to the flat geometry of the OLED device. Typically, only about 50% of the generated light enters the glass substrate and only about 20% escapes into the air. The remaining 50% are trapped in waveguide modes which propagate in the high-index organic layers and the indium-tin-oxide (ITO) electrode. So far, the light trapped in these modes was particularly difficult to be extracted in an economic way. Special substrates with gratings to scatter the trapped light have been used to improve the light out-coupling, where the ITO electrode and the OLED layer stack are deposited on top of such a structure. Such structures will introduce a morphology into the OLED layer stack with decreases the short resistance of the resulting non-flat layer stack compared to OLED devices with a flat OLED layer stack. Structured substrates will additionally increase the manufacturing effort and the manufacturing costs.
Document WO 2010/064186 A1 discloses organic light-emitting device, where the light-reflective layer (electrode on the backside of the OLED device) comprises deformations leading to scattered light enhancing the light out-coupling from the OLED device. These deformations are generated via a laser beam impinging on the rear surface of the light-reflective layer. The laser beam introduces heat to the layer system comprising the organic layers and the reflective layer. When heating the layer stack with this laser beam above the glass transition temperature of the organic layers sandwiched between both electrodes, the whole layer stack forms buckles or wrinkles. The formation of these buckles or wrinkles is driven by the stress in the metal overcoat (reflective layer), which is relieved by wrinkling when the organic layer underneath the reflective layer becomes soft due to heating. These wrinkles are quasi-periodic with a broad periodicity distribution and a directional randomness and act as a type of Bragg grating enhancing the light out-coupling. However, OLED devices treated above the glass transition temperature are mostly shortened, since at the occurring valley bottoms of the wrinkles the distance between cathode and anode is very short leading to high current densities. Sometimes, both electrodes may come into contact directly. Therefore OLED devices treated as described above can only be manufactured with a low production yield and are no reliable over lifetime due to the large probability of occurring shorts. There is a demand to provide an OLED device with improved light out-coupling, which can be manufactured easy and reliable at low costs.